Clarify zero-value behavior of ctlz/cttz intrinsics.

[frewsxcv]

Fixes#34381.

[rust-highfive]

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[frewsxcv]

Hold off on reviewing. Going to discuss this at this week's docs team meeting.

[aturon]

@frewsxcv Ok, let me know when you're ready.

[nagisa]

First of all, underlying LLVM intrinsic has a prototype that looks like this:

declare iXYZ @llvm.ctlz.iXYZ(iXYZ <src>, i1 <is_zero_undef>)

so comparing it to the underlying LLVM intrinsic is not quite right :)

[frewsxcv]

I could just ditch this sentence and not talk about LLVM but leave the x = 0 example.

[frewsxcv]

Removed that sentence in the latest force push.

[frewsxcv]

@aturon@nagisa This is ready for review now.

[pnkfelix]

@bors r+ rollup

[bors]

📌 Commit a99f70b has been approved by pnkfelix

[bors]

⌛ Testing commit a99f70b with merge bd16aa0...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing bd16aa0 to master...